Hi Again phil
The monster ones will foul the swing arm as the rear pipe protrudes further from the manifold before heading south between the swing arm.
The heads cant flow enough to make the big bore downies viable. If you get a chance take a peek at senna 3s bike, he has welded up an underseat pipe on his, Flowed it isnt and he still has the same BHP as me running std downies.
To be honest i dont think there is much to be gained messing with headers untill the head is sorted.
As ali says, I will be selling my BB kit at the end of this season if rumour controll is correct. I would rather have just as much fun on a 583 mucho cheaper.
Glyn